This invention relates to an amplifier type solid state imaging apparatus formed on a silicon substrate and, more particularly, it relates to a solid state imaging device wherein each unit cell has a photodiode processed for device separation by means of a silicon oxide film formed by oxidizing the silicon substrate. The present invention also relates to a solid state imaging apparatus wherein the semiconductor substrate of the apparatus has for each unit cell a region located at a position deeper than the depletion layer region operating as a photodiode, in which region the impurity concentration of the semiconductor slowly increases as a function of the depth in the substrate and part of the signal charges generated in the semiconductor substrate are collected by a signal storage to provide a high dynamic range. The present invention further provides a method of manufacturing such a solid state imaging apparatus and a video system realized by using such a solid state imaging apparatus.
Solid state imaging apparatuses comprising an amplifier type sensor have been developed in recent years. Such apparatuses are featured by detecting optical signals by means of a photoelectric converter/storage and amplifying them in the vicinity of the photoelectric converter/storage.
An amplifier type MOS sensor typically comprises in each unit pixels or unit cell thereof a photodiode and amplifying means including an amplifier transistor for amplifying the signal charges photoelectrically converted and collected by the photodiode in the silicon substrate.
FIG. 1 of the accompanying drawings schematically illustrates in cross section part of a unit cell of a known amplifier type MOS sensor. As seen from FIG. 1, an n-type layer region 12 that constitutes a photodiode with a silicon substrate (p-type layer region) 10 is formed in an oxide film for device separation in a self-aligning manner. A device separating region 16 arranged on a p+-layer 14 is a silicon oxide film formed by oxidizing part of the silicon substrate 10, which silicon oxide film is normally referred to as LOCOS (LOCal Oxidation of Silicon). Reference numerals 18 and 20 in FIG. 1 denote respectively a contact region and a wiring layer connected to the contact region 18, whereas reference numerals 22 and 24 denote respectively the gate of a read-out transistor and a planarizing layer.
The silicon substrate 10 is apt to become defective at and near the corresponding end of the LOCOS region 16 due to the stress generated during the local oxidation. The defect, if any, by turn gives rise to an electric current that appears as leak current of the photodiode.
Now, this problem will be discussed by referring to FIG. 2 of the accompanying drawing.
FIG. 2 is an enlarged cross sectional view showing the boundary of the photodiode and the LOCOS region of FIG. 1. As shown in FIG. 2, a depletion region 26 is formed around the n-type layer region 12 and a depleted region with a large number of defects (multi-defect region) 28 is formed in a lower boundary area of the LOCOS region 16 located adjacent to the n-type layer region 12. Thus, a large number of electron/hole pairs will be generated by heat via the defect levels in the silicon band gap in the multi-defect regions. Then, electrons can transfer into the photodiode to appear as leak current of the photodiode, which leak current can reduce the sensitivity or the S/N ratio of the solid state imaging apparatus.
Thus, since a photodiode and a photodiode are formed in a self-aligning manner in known solid state imaging apparatus, they are accompanied by the problem of leak current on the part of the photodiode generated due to the defect at and near the corresponding end of the LOCOS region 16.
Therefore, the object of the present invention is to provide a solid state imaging apparatus that can prevent any leak current from appearing in the photodiode and is resistive against degradation in the sensitivity, a method of manufacturing such a solid state imaging apparatus, and a video system using such a solid state imaging apparatus.
According to a first aspect of the invention, the above object is achieved by providing a solid state imaging apparatus comprising for each unit cell a substrate of a first conductivity type, at least a first impurity layer of a second conductivity type different from the first conductivity type formed in a surface area of the substrate for a photodiode for forming a photoelectric conversion region, a device separation region for the device separation of the photodiode, the device separation region having a second impurity layer formed in a lower area thereof, and means for amplifying the signal charges collected by the photodiode, wherein the first impurity layer is separated from the device separation region by a predetermined distance.
According to a second aspect of the invention, there is provided a solid state imaging apparatus comprising for each unit cell a substrate of a first conductivity type, at least a first impurity layer of a second conductivity type different from the first conductivity type formed in a surface area of the substrate for a photodiode for forming a photoelectric conversion region, a device separation region for the device separation of the photodiode, the device separation region having a second impurity layer formed in a lower area thereof, and means for amplifying the signal charges collected by the photodiode, wherein the first impurity layer is located adjacent to the second impurity layer and the apparatus further comprises for each unit cell a third impurity layer located adjacent to the second impurity layer and formed continuously at least in part with the surface area of the first impurity region, the third impurity layer having an impurity concentration greater than the second impurity layer.
According to a third aspect of the invention, there is provided a solid state imaging apparatus comprising for each unit cell a substrate of a first conductivity type, at least a first impurity layer of a second conductivity type different from the first conductivity type formed in a surface area of the substrate for a photodiode for forming a photoelectric conversion region, a device separation region for the device separation of the photodiode, the device separation region having a second impurity layer formed in a lower area thereof, and means for amplifying the signal charges collected by the photodiode, wherein the second impurity layer contains a third impurity layer of the first conductivity type located on the side of the first impurity layer and has an impurity concentration greater than the second impurity layer and the first impurity layer is located adjacent to the third impurity layer.
According to a fourth aspect of the invention, there is provided a method of manufacturing a solid state imaging apparatus comprising a first step of forming for each unit cell a first impurity layer in a surface area of a substrate of a first conductivity type by implanting ions of an impurity of a first conductivity type into the substrate to an impurity concentration level higher than the substrate, using a silicon nitride film formed on the substrate as a mask, a second step of forming a device separation region on the first impurity layer, a third step of forming a second impurity layer of a second conductivity type in a surface area of the substrate and a third impurity layer of the second conductivity type in another surface area of the substrate separated from the device separation region by a predetermined distance by implanting ions of an impurity of the second conductivity type different from the first conductivity type, using an electrode formed on the substrate and a resist layer formed on the substrate and the device separation region as a mask and a fourth step of forming a wiring layer on the second impurity layer after removing the resist layer.
According to a fifth aspect of the invention, there is provided a method of manufacturing a solid state imaging apparatus comprising a first step of forming for each unit cell a first impurity layer in a surface area of a substrate of a first conductivity type by implanting ions of an impurity of a first conductivity type into the substrate to an impurity concentration level higher than the substrate, using a silicon nitride film formed on the substrate as a mask, a second step of forming a device separation region on the first impurity layer, a third step of forming a second impurity layer of a second conductivity type in a surface area of the substrate and a third impurity layer of the second conductivity type in another surface area of the substrate adjacent to the device separation region by implanting ions of an impurity of the second conductivity type different from the first conductivity type, using an electrode formed on the substrate and a resist layer formed on the substrate and the device separation region as a mask, a fourth step of forming a fourth impurity layer by implanting ions of the impurity of the first conductivity type into part of the surface area of the third impurity layer adjacent to the first impurity layer to an impurity concentration level higher than the first impurity layer, using the electrode formed on the substrate and the resist layer formed on the substrate and the device separation region as a mask and a fifth step of forming a wiring layer on the second impurity layer after removing the resist layer.
According to a sixth aspect of the invention, there is provided a method of manufacturing a solid state imaging apparatus comprising a first step of forming for each unit cell a first impurity layer in a surface area of a substrate of a first conductivity type by implanting ions of an impurity of a first conductivity type into the substrate to an impurity concentration level higher than the substrate, using a silicon nitride film formed on the substrate as a mask, a second step of forming a device separation region on the first impurity layer, a third step of forming a second impurity layer of a second conductivity type in a surface area of the substrate and a third impurity layer of the second conductivity type in another surface area of the substrate adjacent to the device separation region by implanting ions of an impurity of the second conductivity type different from the first conductivity type, using an electrode formed on the substrate and a resist layer formed on the substrate and the device separation region as a mask, a fourth step of forming a fourth impurity layer by implanting ions of the impurity of the first conductivity type into a surface area of the third impurity layer to an impurity concentration level higher than the first impurity layer, using the electrode formed on the substrate and the resist layer formed on the substrate and the device separation region as a mask and a fifth step of forming a wiring layer on the second impurity layer after removing the resist layer.
According to a seventh aspect of the invention, there is provided a method of manufacturing a solid state imaging apparatus comprising a first step of forming for each unit cell a first impurity layer in a surface area of a substrate of a first conductivity type by implanting ions of an impurity of a first conductivity type into the substrate to an impurity concentration level higher than the substrate, using a silicon nitride film formed on the substrate as a mask, a second step of forming a device separation region on the first impurity layer, a third step of forming a second impurity layer of a second conductivity type in a surface area of the substrate and a third impurity layer of the second conductivity type in another surface area of the substrate separated from the device separation region by a predetermined distance by implanting ions of an impurity of the second conductivity type different from the first conductivity type, using an electrode formed on the substrate and a resist layer formed on the substrate and the device separation region as a mask, a fourth step of forming a fourth impurity layer by implanting ions of the impurity of the first conductivity type into a surface area of the third impurity layer and a surface area of the substrate adjacent to the first impurity layer to an impurity concentration level higher than the first impurity layer, using the electrode formed on the substrate and the resist layer formed on the substrate and the device separation region as a mask and a fifth step of forming a wiring layer on the second impurity layer after removing the resist layer.
According to an eight aspect of the invention, there is provided a method of manufacturing a solid state imaging apparatus comprising a first step of forming for each unit cell a first impurity layer in a surface area of a substrate of a first conductivity type by implanting ions of an impurity of a first conductivity type into the substrate to an impurity concentration level higher than the substrate, using a silicon nitride film formed on the substrate as a mask, a second step of forming a second impurity layer by implanting ions of the impurity of the first conductivity type into a surface area of the substrate adjacent to the first impurity layer to a concentration level higher than the first impurity layer, using the silicon nitride film and a resist layer formed on part of the first impurity layer as a mask, a third step of forming a device separation region on the first impurity layer, a fourth step of forming a third impurity layer of a second conductivity type in a surface area of the substrate and a fourth impurity layer of the second conductivity type in another surface area of the substrate adjacent to the device separation region by implanting ions of an impurity of the second conductivity type different from the first conductivity type, using an electrode formed on the substrate and a resist layer formed on the substrate and the device separation region as a mask and a fifth step of forming a wiring layer on the second impurity layer after removing the resist layer.
According to a ninth aspect of the invention, there is provided a method of manufacturing a solid state imaging apparatus comprising a first step of forming for each unit cell a first impurity layer in a surface area of a substrate of a first conductivity type by implanting ions of an impurity of a first conductivity type into the substrate to an impurity concentration level higher than the substrate, using a silicon nitride film formed on the substrate as a mask, a second step of forming a device separation region on the first impurity layer, a third step of forming a second impurity layer of a second conductivity type in a surface area of the substrate and a third impurity layer of the second conductivity type in another surface area of the substrate separated from the device separation region by a predetermined distance by implanting ions of an impurity of the second conductivity type different from the first conductivity type, using an electrode formed on the substrate and a resist layer formed on the substrate and the device separation region as a mask, a fourth step of forming a fourth impurity layer by implanting ions of the impurity of the first conductivity type into part of the surface area of the third impurity layer adjacent to the first impurity layer to an impurity concentration level higher than the first impurity layer, using the electrode and the resist layer formed on the substrate and the device separation region as a mask and a fifth step of forming a wiring layer on the second impurity layer after removing the resist layer.
According to a tenth aspect of the invention, there is provided a video system comprising an optical system for taking an optical image of a scene and leading the optical image to a predetermined location, an imaging means comprising an array of pixels, each having at least a photodiode region for a photoelectric conversion, a device separation region for the device separation of the photodiode and means for amplifying the signal charges collected by the photodiode, for photoelectrically transforming the optical image led to the predetermined location into an electric signal representing the quantity of light of the optical image on a pixel by pixel basis and signal processing means for storing the electric signal produced by the photoelectric transformation by the imaging means, wherein the imaging means comprises for each cell a device separation region for the device separation of the photodiode, the device separation region being provided with a first impurity layer of a first conductivity type formed in a lower area thereof and a second impurity layer of a second conductivity type different from the first conductivity type formed in a surface area of the substrate of the first conductivity type separated from the device separation region by a predetermined distance.
According to an eleventh aspect of the invention, there is provided a video system comprising an optical system for taking an optical image of a scene and leading the optical image to a predetermined location, an imaging means comprising an array of pixels, each having at least a photodiode region for a photoelectric conversion, a device separation region for the device separation of the photodiode and means for amplifying the signal charges collected by the photodiode, for photoelectrically transforming the optical image led to the predetermined location into-an electric signal representing the quantity of light of the optical image on a pixel by pixel basis and signal processing means for storing the electric signal produced by the photoelectric transformation by the imaging means, wherein the imaging means comprises for each cell a device separation region for the device separation of the photodiode, the device separation region being provided with a first impurity layer of a first conductivity type formed in a lower area thereof, a second impurity layer of a second conductivity type different from the first conductivity type formed in a surface area of the substrate of the first conductivity type and a third impurity layer of the first conductivity type at least in part of the surface area of the second impurity layer and having an impurity concentration greater higher than the first impurity layer.
According to a twelfth aspect of the invention, there is provided a video system comprising an optical system for taking an optical image of a scene and leading the optical image to a predetermined location, an imaging means comprising an array of pixels, each having at least a photodiode region for a photoelectric conversion, a device separation region for the device separation of the photodiode and means for amplifying the signal charges collected by the photodiode, for photoelectrically transforming the optical image led to the predetermined location into an electric signal representing the quantity of light of the optical image on a pixel by pixel basis and signal processing means for storing the electric signal produced by the photoelectric transformation by the imaging means, wherein the imaging means comprises for each cell a device separation region for the device separation of the photodiode, the device separation region being provided with a first impurity layer of a first conductivity type having an impurity concentration higher than the first impurity layer formed in a lower area thereof and a third impurity layer of a second conductivity type different from the first conductivity type formed adjacently relative to the second impurity layer in a surface area of the substrate of the first conductivity type.
According to the invention, the photodiode is separated from the corresponding end of the LOCOS region containing a large number of defects therein, which is made to show a high concentration level of an impurity having an conductivity type opposite to that of the photodiode.
According to the invention, the conventional completely depleted structure of the photodiode is modified within an area where incident light is absorbed and attenuated but remains effective so as to arrange a completely depleted region and an undepleted signal storage region within the effective stroke of incident light and provide the impurity concentration of the undepleted semiconductor impurity layer with a gradient slowly rising toward the adjacent pixel or the bottom of the substrate so that the signal charges generated in the undepleted region are encouraged to transfer into the storage side due to the concentration gradient and become distributed and only part of the signal charges may be stored.
According to the invention, the impurity concentration of the impurity semiconductor surrounding the photodiode is provided with a mild gradient to effectively collect the signal charges obtained by the photoelectric conversion of incident light and discard part of the large volume of signals generated by very bright light as waste.
According to the invention, the impurity concentration of the semiconductor substrate with a gradient slowly rising as a function of the depth of the substrate in an area located below the depleted region of the photodiode to effectively collect the signal charges obtained by the photoelectric conversion of incident light. As a result, only part of the signal charges generated by very bright light may be stored, the rest being discharged to the substrate side by diffusion to realize a high dynamic range.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.